Fleas are tiny and move very quickly! You can tell if Flash has them if there little black dots, like black dandruff, on his bedding or if you comb him. There are many types of treatment available from sprays and shampoos to tablets and spot on. You can also buy collars impregnated with flea killer. Personally I use a spot on. I don't think the collars are very effective and cats are not too keen on sprays usually!
It's a good idea to treat bedding too. You can buy sprays to treat household surfaces.